Miniature button alkaline metal oxide cells, such as alkaline silver oxide cells, have gained wide commercial acceptance for many applications because they are characterized as being high capacity, small volume electric cells. In other words, they have a high power output and energy per unit weight and unit volume of active cathode material. One of the major disadvantages of divalent silver oxide cells is that they discharge at two successive different potentials. This is due to the fact that the active materials of such cells are first divalent silver oxide (AgO) which is then reduced to monovalent silver oxide (Ag.sub.2 O). Silver oxide cells using monovalent silver oxide as the only active cathode material will have a theoretical unipotential discharge at about 1.57 volts but the capacity in milliampere hours per gram of monovalent silver oxide is substantially lower than the capacity with divalent silver oxide. On the other hand, silver oxide button cells [0.455 inch (1.16 cm diameter) by 0.210 inch (0.533 cm high)] using only divalent silver oxide as the starting active cathode material will discharge at a first potential at about 1.7 volts across a 300-ohm resistor for the first 40 hours of discharge, for example, and then drop to approximately 1.5 volts for the balance of the useful discharge life. Thus, monovalent silver oxide cells having the advantage of discharging at a single unipotential plateau but with some sacrifice in capacity as compared with divalent silver oxide cells which have the advantage of having a much higher capacity but with the disadvantage of discharging at two successive distinct voltage plateaus. Divalent silver oxide has about 1.9 times more capacity per gram than monovalent silver oxide and about 2 times more capacity per unit volume than monovalent silver oxide.
Many cell or battery applications, particularly in transistorized devices such as hearing aids, watches, calculators and the like, require an essentially unipotential discharge source for proper operation and, therefore, cannot effectively use the dual voltage level discharge which is normally characteristic of divalent silver oxide cells.
Consequently, many methods have been proposed for obtaining a unipotential discharge from a divalent silver oxide cell without undue sacrifice in capacity. One method disclosed in U.S. Pat. Nos. 3,615,858 and 3,655,450 entails providing a continuous layer of monovalent silver oxide in physical and electrical contact with a divalent silver oxide pellet. During assembly of the cell, the cathode pellet is disposed against the inner surface of a cathode cup or collector whereupon the layer of monovalent silver oxide physically isolates the divalent silver oxide from contact with the cathode cup so that the sole electronic path for discharge of the divalent silver oxide is through the monovalent silver oxide layer.
In U.S. Pat. No. 3,476,610 a silver oxide battery is disclosed which employs a positive electrode comprised mainly of divalent silver oxide with the addition of monovalent silver oxide present in part as an electrolyte-impermeable masking layer. This layer isolates the divalent silver oxide from contact with the electrolyte of the battery until discharge begins whereupon the monovalent silver oxide becomes electrolyte-permeable. When this occurs, the electrolyte then begins to contact the divalent silver oxide. In addition, the monovalent silver oxide is also present as an interposed layer between the divalent silver oxide and the inner surface of the cathode cup or collector so as to isolate the divalent silver oxide from electronic contact with said cathode cup which is the positive terminal of the cell.
In U.S. Pat. No. 3,484,295 a silver oxide battery is disclosed which utilizes a positive silver electrode comprising divalent silver oxide and monovalent silver oxide. The latter oxide is employed as an electrolyte-impermeable layer which is interposed between the divalent silver oxide and the battery components containing the electrolyte so as to isolate the divalent silver oxide from contact with the electrolyte until the monovalent silver material is discharged. If the discharge product of the monovalent silver material is continually reoxidized by the divalent silver material in the presence of the battery electrolyte, then it is possible that the battery will yield a unipotential discharge.
In U.S. Pat. No. 3,920,478 a silver oxide cell is disclosed which employs a positive electrode comprising divalent silver oxide housed in a positive cathode container, and interposed between the positive electrode and the inner wall of the cathode container and/or between the positive electrode and the separator is a discontinuous oxidizable metal, such as a zinc screen, which functions to reduce a portion of the divalent silver oxide to monovalent silver oxide which isolates the divalent silver oxide portion of the positive electrode from the container so as to produce a unipotential discharge on low drain conditions.
In U.S. Pat. No. 3,925,102 a silver oxide cell is disclosed which employs a positive electrode comprising divalent silver oxide housed in a positive electrode container having an upstanding wall and a closed end. Interposed between the positive electrode and the inner upstanding wall is an oxidizable zinc ring which functions to reduce a portion of the divalent silver oxide to monovalent silver oxide which isolates the divalent silver oxide portion of the positive electrode from the container so as to produce a unipotential discharge on low drain conditions.
The silver oxide electrodes for use in the above-described cells are generally molded into inflexible pellet-type solid electrodes with the aid of a lubricant. The presence of many of the conventional lubricants, such as graphite, in molded silver oxide electrodes containing divalent silver oxide has been found to adversely affect the shelf life of the cells employing such electrodes and/or the unipotential discharge characteristic of the cells.
Accordingly, it is an object of the present invention to provide a metal oxide electrode for electrochemical cells which comprises a metal oxide and a minor amount of solid ethylene acrylic acid polymer.
Another object of this invention is to provide a silver oxide electrode for electrochemical cells which comprises divalent silver oxide and a minor amount of an ethylene acrylic acid polymer.
Another object of this invention is to incorporate a minor amount of an ethylene acrylic acid polymer into a divalent silver oxide-containing material so as to provide lubricity and cohesion to the mixture so that said mixture can be easily molded into a substantially cohesive inflexible body which can be easily handled.
Another object of this invention is to provide a porous positive electrode for silver oxide cells which comprises divalent silver oxide and a minor amount of an ethylene acrylic acid polymer and which electrode will exhibit good electrolyte absorption characteristics.
Another object of this invention is to provide a method for producing a molded metal oxide electrode.